1. Field of the Invention
The present invention relates to a rapidly quenched metal strip for use as a core material for transformers, magnetic shields, choke coils, etc.
2. Description of the Prior Art
A centrifugal rapid quenching process, a single roll process, a twin roll process, and the like process have been known as processes for continuously producing a metal strip by rapidly quenching an alloy from a molten state. These process produce a strip or wire by injecting, from an orifice or the like, a molten metal on the inner or outer periphery of a metal drum, rotating at high speed, to rapidly solidify the molten metal. Moreover, an amorphous alloy similar to a liquid metal can be obtained by properly selecting the alloy composition.
Since amorphous alloys have distinguishing properties, attention has been paid thereto from a practical standpoint. Some of the alloys have already been used in practice. In particular, an Fe-based amorphous alloy has been used as the core material of power transformers and high frequency transformers.
Very pure raw materials have heretofore been used as raw materials for an alloy for producing such a rapidly quenched metal strip. For example, electrolytic iron and ferroboron of high grade have been used as an iron source and a boron source, respectively. The very pure raw materials have been used to avoid adverse effects such as deterioration of magnetic properties caused by impurities contained in raw materials of low grade.
Many studies on the impurities have been reported. For example, Kokai (Japanese Unexamined Patent Publication) No. 57-137451 discloses the maximum allowable contents of impurities in amorphous Fe--Si--B alloy strips. An example of the maximum allowable contents of impurities in an Fe-15.3% B-5.8% Si (by at %) alloy is as follows: 0.14% by weight of Mn, 0.014% by weight of S and 0.005% by weight of P. In order to satisfy the allowable ranges, very pure raw materials must be used. Consequently, the production cost has become high. Several studies have been reported on deterioration of the properties such as magnetic properties and mechanical properties. For example, it has been elucidated that Al contained as an impurity deteriorates the properties because crystallization on the strip surface (e.g., C. Kaido et al.: Rapidly Quenched Metals IV vol. 2 (1981) 957)).
However, the crystallization alone cannot account for the deterioration of the mechanical properties of rapidly quenched metal strips. Even when rapidly quenched metal strips are produced from highly pure raw materials, the following phenomena have often taken place: during casting and coiling a strip, the strip is suddenly fractured; marked variations in mechanical properties, for example, the standard deviation .sigma. of bending fracture strain .epsilon..sub.f of a strip exceeds 50% of the average value &lt;.epsilon..sub.f &gt; among lots or depending on sites within a lot. When a strip is used as cores in the production of transformers, the strip is required to be as long as from several meters to several tens of meters. When the strip shows variations in the mechanical properties within a lot, it is very difficult to use only the portion of the strip having excellent mechanical properties while the portion of the strip having poor mechanical properties is removed by cutting. Accordingly, there has been no highly reliable strip showing such decreased variations in the mechanical properties that it shows substantially no fracture during the production of transformers. Moreover, the fracture of the strip lowers its yield in the production of transformers, and causes an increase in the cost.
There has been substantially no report that nonmetallic inclusions exist in rapidly quenched metal strips. Moreover, there has been no report that discloses the influence of the nonmetallic inclusions on the mechanical properties such as a bending strength and the magnetic properties of the strips. There is substantially only one report (H. C. Fiedler et al.: J. Magn. Magn. Mat., 26 (1982) 157) that discloses nonmetallic inclusions in strips. According to the report, crystallization is caused by Al oxides. On the other hand, when the conventional materials were viewed from the standpoint of rapidly quenched metal strips, it was found that there are some strips having a thickness level of, for example, 25 .mu.m in which the density of nonmetallic inclusions (e.g., oxides mainly containing Fe oxides, or oxides mainly containing Al oxides or Si oxides) present is about 10.sup.7 nonmetallic inclusions/mm.sup.3 and in which the particle size distribution of the nonmetallic inclusions is up to 3 .mu.m, that is, the proportion of fine nonmetallic inclusions is overwhelmingly large. However, in most of the strips, the proportion of the nonmetallic inclusions and the particle size distribution are in completely random states.